Warnings about VariationPath overrides when none are used

[alexander-held]

The

variation override specified, but {VariationPath} not found in default path

warning message triggers for histogram inputs even when no overrides are used at the moment.

Example test that will currently fail for test__histo_path:

# no variation path anywhere, so no warning
assert (
    collector._histo_path(
        "f.root:h1", "", {}, {}, {}, None
    )
    == "f.root:h1"
)
assert (
    "variation override specified, but {VariationPath} not found in default path"
    not in [rec.message for rec in caplog.records]
)
caplog.clear()

This needs to be fixed, presumably in templates.collector._histo_path.